1. Technical Field
The present disclosure relates to an illumination device and to a projector including the illumination device.
2. Description of the Related Art
An illumination device which emits coherent light, such as laser light, has the problem of the formation of speckles. The “speckles” refer to a speckle pattern that may appear on a scattering surface when it is irradiated with coherent light such as laser light. Speckles, when formed on a screen, are observed as speckled uneven luminance, in other word uneven brightness, which can have a physiologically harmful influence on the viewer. In order to solve the speckle problem, JP 2012-237813A has proposed an illumination device in which the incident angle of illumination light entering a scattering surface changes with time. Speckles on the scattering surface, caused by diffusion of coherent light, are therefore superimposed and time-averaged, whereby the speckles are obscured.
In particular, the illumination device disclosed in JP 2012-237813A includes a light source for emitting coherent light, a first lens array, a second lens array and a condenser lens, which are arranged in this order along the light path. The light source irradiates the first lens array with a parallel light flux in a constant direction in such a manner that the irradiating light scans the first lens array. Light that has exited a unit lens contained in the first lens array enters a corresponding unit lens of the second lens array; and such lights are condensed by the condenser lens into an illumination region. In the illumination device disclosed in JP 2012-237813A, a position on the condenser lens, at which coherent light leaves the lens for the illumination region, changes with time. Accordingly, the illumination region is to be illuminated from various directions corresponding to various regions of the optical element.
As shown in FIG. 10, in the illumination device disclosed in JP 2012-237813A, light that has entered a unit lens 156 of a first lens array 155 focuses on a unit lens 161 of a second lens array 160, corresponding to the unit lens 156, and the light is directed by a condenser lens 165 toward an illumination region LZ. Particularly in the illumination device 140, light from the light source enters each unit lens 156 of the first lens array 155 from a constant direction. Therefore, light that has entered each unit lens 156 of the first lens array 155 passes through a focal point fp located on a unit lens 161 of the second lens array 160, corresponding to the unit lens 156. The illumination device makes it possible to use light from the light source at a high efficiency. Coherent light, which can cause speckles, is especially excellent in the spatial coherency. Therefore, when coherent light is used, light entering the second lens array 160 can be efficiently collected on the focal points fp on the unit lenses 161. The use of coherent light is thus preferred from the viewpoint of light use efficiency.
However, in the illumination device disclosed in JP 2012-237813A, light beams that have entered a unit lens 156 of the first lens array 155 all pass through a predetermined position on the second lens array 160, namely, the focal point fp of the unit lens 156. Accordingly, the incident direction of light, which has exited the condenser lens 165 and enters the illumination region LZ, changes stepwise, not continuously. It is therefore possible that speckles may not be obscured effectively especially when using coherent light which can be efficiently collected on the focal points fp on the unit lenses 161.
Further, in the case where a region of the first lens array 155 which coherent light enters at a certain moment, i.e. a light spot on the first lens array 155, formed by coherent light from the light source, is smaller than the size of each unit lens 156 as shown in FIG. 11, light from the light source enters only part of a unit lens 156 contained in the first lens array 155 and, in addition, there exists a time period during which light scans the unit lens 156. During the time period, the illumination region L is irradiated with light only from an approximately constant direction. It is highly possible in this case that the speckle reduction effect will be low.